The present invention relates to an interconnection system formed by chemical vapor deposition (CVD) and a method for producing the same.
This application is based on Japanese Patent Application No. Hei 10-65781.
In the manufacturing of semiconductor circuits, an interconnection film is formed on a semiconductor substrate to connect active and passive elements.
The interconnection film is manufactured, in general, by a sputtering process in which particles are produced in a vacuum and are deposited onto the substrate, forming the thin film.
In recent years, as integration of circuits has progressed rapidly, aspect ratios of contact holes and through holes have increased, and the sputtering process is often unsatisfactory and ineffective to fill or plug the holes. There is a need to develop another process for manufacturing an interconnection film.
In addition to the sputtering process, chemical vapor deposition (CVD) has been found to be a convenient and effective process for forming interconnection films.
This process usually involves nucleating tungsten in holes (recesses) extending through an insulative layer, depositing a film to plug holes based on the tungsten nucleation, and depositing an upper layer onto the plugging film.
To performing the step of forming the upper layer onto the plugging film after plugging the holes in the insulative layer with tungsten film, a single-wafer type tungsten CVD device utilizes, in general, a single chamber.
The process for depositing the tungsten film using the single-wafer type tungsten CVD device involves introducing WF6 as a reactant gas to deposit the film plugging the holes, and subsequently introducing WF6 into the chamber to deposit the upper tungsten interconnection film onto the plugging film.
The present inventor has found, based on measurements of the warp of the semiconductor substrate using a stress measuring device, that stress occurs on the interconnections of the semiconductor substrate. The reflectance on the surface of the film measured by a reflectometer is low, and irregularities are detected on the surface of the film with the decreased reflectance by a SEM (scanning electron microscope).
After technically analyzing the above problem, the present inventor has reached the conclusion that when the semiconductor substrate warps due to the stress on the interconnections, the surface smoothness of the film which is to be a base for elements formed on the substrate is degraded, resulting in undesirable effects on the characteristics of the manufactured semiconductor device.
The reduction of the reflectance is caused by the irregularities on the surface of the upper interconnection film, which make the film thickness uneven, and in the following etching process residues or thinning may occur in the interconnection film, which may cause short-circuits.
The technical analysis by the present inventor shows that these problems are caused by fluorine which is dissociated from WF6 and contaminates the interconnection film.
The Japanese Patent Application, First Publication No. Hei 7-111253 discloses a technique for depositing a tungsten film onto an insulative layer, which involves stopping supply of a reactant gas between the first and second steps while cleaning the surface of the film by flowing only a carrier gas.
This technique, however, merely cleans the surface of the film and does not control the concentration of the impurities, and it is therefore impossible to prevent the stress on the interconnection film and the reduction of the reflectance due to the impurities in the film. The problems in the background process for forming the interconnection film using the CVD remain.
It is therefore an object of the present invention to provide an interconnection system and a method for producing the same, which can avoid contamination of impurities dissolved from a reactant gas into an interconnection film, and which can improve the performance of a manufactured semiconductor device.
In order to accomplish the above object, the interconnection system of the present invention comprises an interconnection film formed by chemical vapor deposition, wherein the interconnection film comprises an upper layer and a lower layer in which the concentrations of impurities are different.
The concentration of the impurities in the upper layer is less than the concentration of impurities in the lower layer. Preferably, the concentration of the impurities in the upper layer is in a range of {fraction (1/7)} to ⅕ of the concentration of the impurities in the lower layer. The impurities in the interconnection film are components dissociated from a reactant gas in a deposition process of the interconnection film. Increasing stress and reduced reflectance of said interconnection film are results of the impurities.
The interconnection film is formed by a two step process which involves depositing the lower layer and subsequently depositing the upper layer thereon. The lower layer fills a recess, and the upper layer is formed onto the lower layer.
The interconnection film is formed of tungsten using of WF6 a reactant gas. The tungsten interconnection film contains the impurities of fluorine dissociated from the WF6 gas. The concentration of the fluorine in the upper layer is in a range of {fraction (1/7)} to ⅕ of the concentration of the fluorine in the lower layer.
In another aspect of the present invention, a method for producing an interconnection film comprising an upper layer and a lower layer by chemical vapor deposition using a single chamber, comprises: a lower layer forming step of depositing the lower layer in a recess by evacuating the chamber and by injecting a reactant gas into the chamber; a cleaning step of subsequently reducing a partial pressure of impurities which are dissociated from the reactant gas; and an upper layer forming step of subsequently depositing an upper layer onto the lower layer by injecting a reactant gas into the chamber.
The cleaning step comprises reducing the concentration of the impurities in the upper layer. The upper layer and the lower layer are made of the same materials. The concentration of the impurities in the upper layer is in a range of {fraction (1/7)} to ⅕ of the concentration of the impurities in the lower layer.
In another aspect of the present invention, a method for producing an interconnection film comprising an upper layer and a lower layer by chemical vapor deposition using a single chamber, comprises: a lower layer forming step of depositing the lower layer in a recess, where tungsten is nucleated, by evacuating the chamber and by injecting a WF6 reactant gas into the chamber; a cleaning step of subsequently reducing a partial pressure of fluorine which is dissociated from the WF6 reactant gas and stays in the chamber; and an upper layer forming step of subsequently depositing an upper layer onto the lower layer by injecting a WF6 reactant gas into the chamber.
The cleaning step comprises reducing the concentration of the fluorine in the upper layer. The concentration of the fluorine in the upper layer is in a range of {fraction (1/7)} to ⅕ of the concentration of the fluorine in the lower layer. The ratio of the flows of the reactant gases in the lower layer forming step and in the upper layer forming step is in a range of approximately 1:0.25 to 1:0.5.
The cleaning step may include injecting only a replacement gas into the chamber. In the cleaning step, the replacement gas is an inert gas or a reducing gas, and the degree of vacuum in the chamber is in a range of 102 to 10xe2x88x922 Torr. The cleaning step may include evacuating the chamber. In the cleaning step, the degree of vacuum is in a range of 10xe2x88x921 to 10xe2x88x923 Torr. The cleaning step may include both evacuating the chamber and injecting a replacement gas into the chamber. In the cleaning step, the degree of vacuum in the chamber is in a range of 10xe2x88x922 to 10xe2x88x921 Torr.
According to the present invention, stress on the interconnection film and reduction of the reflectance can be prevented because the interconnection film is formed while the concentration of the impurities in the upper layer is reduced.
Further, the cleaning process is performed between the lower layer forming step and the upper layer forming step, and the steps are performed consecutively, enhancing process efficiency.